Why won't it let me save my rendered image?
I know a answer, but it may not work.


Set the file output destination and image format as you would normally do.
Edit both starting frame and ending frame of your image to the frame of which you want to render (ex.: I want the frame 260 to be rendered as an image, I put the starting frame and ending frame to 260). You can edit it in the render menu somewhere or in the timeline
Click on the RENDER ANIMATION and not the RENDER IMAGE. Because when you make the file output an image file and put the animation render, you get a image of each frame, thus setting the frame start and end will make it render only that frame. I'm not sure if it will work for you, and it may not apply transparency.